The present invention relates to a color sonar system for displaying a high echo signal and a low echo signal with a predetermined color for each level thereof.
Generally, an ultrasonic wave of a high frequency is reflected by small fish in the surface of the sea and an ultrasonic wave of a low frequency is reflected by large fish in the deep portion of the sea. Therefore, when the low frequency ultrasonic echo signals which are reflected from reflecting surfaces in the sea are subtracted from the high frequency ultrasonic echo signals which are reflected from reflecting surfaces in the sea at the same time, signals of the same level as each other are eliminated. Therefore, because the high and low frequency echo signals which are reflected by the surface and the bottom of the sea are of the same level as each other, these signals are eliminated.
However, the level of the echo signals reflected by a school of small fish are different from the other signals; thus these signals are not eliminated. When the signals which are reflected by the school of small fish are applied to a cathode ray tube, a picture of the school of small fish is displayed on its screen.
On the basis of this principle, a color fish sonar can be provided in which high and low frequency ultrasonic waves are respectively transmitted from high and low frequency transducers to the sea, and the high and low frequency echoes reflected by the bottom of the sea and a school of fish are respectively received by high and low frequency transducers and are converted high and low frequency echo signals respectively. These high and low frequency echo signals are amplified by high and low frequency amplifiers, and the differences between the high and low frequency echo signals at various levels are displayed by a color cathode ray tube.
In this color fish sonar, only the differences between the high and low frequency echo signals are displayed with predetermined colors at each level. Therefore, because the differences between the levels of the high and low frequency echo signals are not displayed, the kind and distribution of fish cannot be correctly discriminated by such a display.